1. Field of the Invention
This invention relates to a Zirlo alloy and to a method for fabricating a Zirlo alloy in tubes or strips. Zirlo is used in the elevated temperature aqueous environment of a reactor of a nuclear plant and is an alloy of primarily zirconium containing nominally by weight 1 percent niobium, 1 percent tin and 0.1 percent iron. Generally, Zirlo comprises 0.5 to 2.0 weight percent niobium, 0.7 to 1.5 weight percent tin and 0.07 to 0.28 of at least one of iron, nickel and chromium and up to 200 ppm carbon. The balance of the alloy comprises essentially zirconium.
2. Background of the Invention
Among the objectives of fabrication methods for Zirlo are obtaining good corrosion resistance with acceptable texture. The relationship between pilger reduction formability and texture parameters are presented below by first describing the formability parameter and then showing the applicability of the formability parameter to pilger reduction.
The formability parameter describes the small and large strain behavior of anisotropic materials such as Zirlo. W. A. Backofen, Deformation Processing, Addison-Wesley Publishing Company, 1972, pp. 85-86. defined the formability parameter B to describe the distortion or anisotropy of the yield locus. Backofen defined the formability parameter as EQU B=.sigma..sub.I /2 .sigma..sub.IV
where .sigma..sub.I is the maximum stress in quadrant I and .sigma..sub.IV represents the shear stress in quadrant IV of the yield locus. The B parameter is important because the higher the B value, the better the material formability. Although the yield behavior is associated with small strains, the formability parameter also describes high strain metalworking operations. For deep cup drawing, the drawing limit is given by the limiting drawing ratio, LDR EQU ln(LDR)=.sigma..sub.w /.sigma..sub.f
where .sigma. is the stress and the subscripts w and f denote the cup wall and flange, respectively. W. F. Hosford and R. M. Caddell, Metal Forming Mechanics and Metallurgy, Prentice-Hall, 1983, pp. 277-279, have shown for deep cup drawing that the formability parameter is related to the LDR according to the equation EQU B=ln(LDR)
Hence, the formability parameter describes deep cup drawing.
Pilger reduction and deep cup drawing are considered to be related processes based on the similarity between the stresses and strains developed during pilgering and deep cup drawing. Pilgering is a direct compression metalworking operation. A force is applied to the tubeshell surface by the die and metal flows at right angles to the applied force. In the case of deep cup drawing, the applied force is tensile, but large compressive forces are developed by the reaction of the workpiece and the die. More specifically, as the metal is inwardly drawn, the outer circumference continually decreases. This means that in the flange region the workpiece is subject to compressive hoop strain and stress. Hence both pilgering and deep cup drawing may be considered to be similar metalworking operations because they both involve large compressive strain and stress.
The texture of anisotropic tubes is characterized by the transverse contractile strain ratios. The transverse contractile strain ratios of an anisotropic tube define the resistance to wall thinning. The transverse contractile strain ratios are EQU R=.DELTA.e.sub..theta. /.DELTA.e.sub.r for .sigma..sub..theta. =.sigma..sub.r =0 EQU P=.DELTA.e.sub.z /.DELTA.e.sub.r for .sigma..sub.z =.sigma..sub.r =0
where .theta., z and r are the hoop, axial and radial directions. K. L. Murty, "Application of Crystallographic Textures of Zirconium alloys in the Nuclear Industry", Zirconium in the Nuclear Industry: Eight International Symposium, ASTM STP 1023, American Society for Testing and Materials, Philadelphia, 1989, pp. 570-595, has developed the relationship between the formability parameter and the contractile strain ratios R and P. The relationship is EQU B=[{(R+1)(R+4RP+P)}/{4R(R+P+1)}].sup.0.5
A pilger reduction operation is considered successful when a defect free tube is produced. The production of a defect free tubeshell depends on whether the hoop and/or axial stress remains below the tensile strength of the metal near the ID surface. When the hoop and/or axial stress exceeds the tensile strength of the metal near the tubeshell ID surface, the tubeshell develops small tears or microfissures. Presumably, an increase in the formability parameter is associated with a decrease in the tendency for microfissure development.